Copy of `Illinois State Geological Survey - Geological terms`

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AGGRADINGOne that is actively depositing sediment in its channel or floodplain because it is being supplied with more load than it can transport.

ALLUVIUMA general term for clay, silt, sand, gravel, or similar unconsolidated sorted or semi-sorted sediment deposited during comparatively recent time by a stream or other body of running water.

ANTICLINEA convex upward rock fold in which strata have been bent into an arch; the strata on each side of the core of the arch are inclined in opposite directions away from the axis or crest; the core contains older rocks than does the perimeter of the structure.

AQUIFERA geologic formation that is water-bearing and which transmits water from one point to another.

ARENITEA relatively clean quartz sandstone that is well sorted and contains less than 10% argillaceous material.

ARGILLACSaid of rock or sediment that contains, or is composed of, clay-sized particles or clay minerals.

AUTHIGENICFormed or generated in place; specif. said of rock constituents and minerals that have not been transported or that crystallized locally at the spot where they are now found, and of minerals that came into existence at the same time as, or subsequently to, the formation of the rock of which they constitute a part. The term, as used, often refers to...

AUXVASESThe Aux Vases Sandstone is named for the Aux Vases River in Ste. Genevieve County, Missouri, and the type section consists of outcrops in the Mississippi River bluffs at the mouth of the Aux Vases River. The Aux Vases consists of sandstone, siltstone, and minor amounts of shale and, locally, dolomite and limestone. It occurs in much of the area of ...

BASELLower limit of erosion of the land's surface by running water. Controlled locally and temporarily by the water level of stream mouths emptying into lakes, or more generally and semi-permanently by the level of the ocean (mean sea level).

BASINA topographic or structural low area that generally receives thicker deposits of sediments than adjacent areas; the low areas tend to sink more readily, partly because of the weight of the thicker sediments; the term also denotes an area of relatively deep water adjacent to shallow-water shelf areas.

BEDA naturally occurring layer of earth material of relatively greater horizontal than vertical extent that is characterized by physical properties different from those of overlying and underlying materials. It also is the ground upon which any body of water rests or has rested, or the land covered by the waters of a stream, lake, or ocean; the bottom...

BRAIDEDA low-gradient, low-volume stream flowing through an intricate network of interlacing shallow channels that repeatedly merge and divide, and are separated from each other by branch islands or channel bars. Such a stream may be incapable of carrying all of its load. Most streams that receive more sediment load than they can carry become braided.

BURLINGThe Burlington Limestone is named for the city of Burlington, Des Moines County, Iowa, where the formation is well exposed and about 70 feet thick. In Illinois, the Burlington extends from Henderson County in the northwest across a roughly triangular area southward to Jackson County and eastward to Iroquois County. Good outcrops are found in the Mi...

CALCARDescribes a limestone composed of more or less worn fragments of shells or pieces of older limestone. The particles are generally sand-sized.

CALCARESaid of a rock containing as much as 50% of calcium carbonate (CaCO3), but also composed of something else (synonym: limy).

CALCINEDThe heating of calcite or limestone to its temperature of dissociation so that it loses its carbon dioxide; also applied to the heating of gypsum to drive off its water of crystallization to make plaster of paris.

CALCITEA common rock-forming mineral consisting of CaCO3; it may be white, colorless, or pale shades of gray, yellow, and blue; it has perfect rhombohedral cleavage, appears vitreous, and has a hardness of 3 on the Mohs' scale; it effervesces (fizzes) readily in cold dilute hydrochloric acid. It is the principal constituent of limes...

CAMBRIANThe earliest period of the Paleozoic, thought to have covered the span of time between 543 and 490 million years ago; also, the corresponding system of rocks. It is named after Cambria, the Roman name for Wales, where rocks of this age were first studied.

CAVEA cavity in the earth large enough for a human to enter. Caves can form as a result of physical and chemical weathering of rock. Physical weathering usually produces shelter-type caves that extend into the rock for only a few feet. Chemical weathering of rock can produce caves (solution channels along fractures and bedding planes) that extend for m...

CENOZOICAn era of geologic time, from the beginning of the Tertiary period to the present. (Some authors do not include the Quaternary, considering it a separate era.) It is characterized paleontologically by the evolution and abundance of mammals, advanced mollusks, and birds; paleobotanically, by angiosperms. The Cenozoic is considered to have begun abou...

CHERTSilicon dioxide (SiO2); a compact, massive rock composed of minute particles of quartz and/or chalcedony; it is similar to flint but lighter in color.

CHESTERThe Chesterian Series, the uppermost series of the Mississippian System, is named for Chester, Randolph County, where it is well exposed in the bluffs of the Mississippi River. The Chesterian Series consists of limestone-shale formations alternating with sandstone-shale formations. It extends from the major unconformity at the base of the Pennsylva...

CHITINOUSChitin: A resistant organic compound with the same basic carbohydrate structure as cellulose, but nitrogenous because some hydroxyl groups are replaced by ascetamide groups. It is a common constituent of various invertebrate skeletons such as insect exoskeletons and foraminiferal inner test, and also occurs in hyphae and spores of fungi.

CLASTICSaid of rocks composed of particles of other rocks or minerals, including broken organic hard parts as well as rock substances of any sort, transported and deposited by wind, water, ice, or gravity.

CLOSEDDA low, roughly concave topographic feature in a landscape. Rain falling within the boundaries of the depression would be channeled toward its lowest part (usually near its center).

CLOSUREThe difference in altitude between the crest of a dome or anticline and the lowest structural or elevation contour that completely surrounds it.

COLUMNARA graphic representation, in the form of one or more vertical column(s), of the vertical succession and stratigraphic relations of rock units in a region.

CONCRETION(a) A hard, compact mass or aggregate of mineral matter, normally subspherical, but commonly oblate, disk-shaped, or irregular with odd or fantastic outlines; formed by precipitation from aqueous solution about a nucleus or center, such as a leaf, shell, bone, or fossil, in the pores of a sedimentary or fragmental volcanic rock, and usually of a co...

CONFORMSaid of strata deposited one upon another without interruption in accumulation of sediment; beds parallel.

CRETACEOUSThe final period of the Mesozoic era (after the Jurassic and before the Tertiary period of the Cenozoic era), thought to have covered the span of time between 144 and 65 million years ago; also, the corresponding system of rocks. It is named after the Latin word for chalk (creta) because of the English chalk beds of this age.

DELTAA low, nearly flat, alluvial land form deposited at or near the mouth of a river where it enters a body of standing water; commonly a triangular or fan-shaped plain extending beyond the general trend of a coastline.

DETRITALPertaining to or formed from detritus; said esp. of rocks, minerals, and sediments. The term may indicate a source outside the depositional basin or a source within it.

DETRITUSLoose rock and mineral material produced by mechanical disintegration and removed from its place of origin by wind, water, gravity, or ice; also, find particles of organic matter, such as plant debris.

DEVONIANA period of the Paleozoic era (after the Silurian and before the Mississippian), thought to have covered the span of time between 400 and 345 million years ago; also, the corresponding system of rocks. It is named after Devonshire, England, where rocks of this age were first studied.

DOLOMITEA mineral, calcium-magnesium carbonate (Ca,Mg[CO3]2); also the name applied to sedimentary rocks composed largely of the mineral. It is white, colorless, or tinged yellow, brown, pink, or gray; has perfect rhombohedral cleavage; appears pearly to vitreous; and effervesces feebly in cold dilute hydrochlo...

DRIFTAll rock material transported by a glacier and deposited either directly by the ice or reworked and deposited by meltwater streams and/or the wind.

DRIFTLESSA 10,000-square-mile area in northeastern Iowa, southwestern Wisconsin, and northwestern Illinois where the absence of glacial drift suggests that the area may not have been glaciated.

ENDMORAINEA ridge or series of ridges formed by accumulations of drift built along the outer margin of an actively flowing glacier at any given time; a moraine that has been deposited at the lower or outer end of a glacier.

ENECHELONAn adjective describing geologic features that are in an overlapping or staggered arrangement. Each is relatively short but collectively they form a linear zone.

EPOCHAn interval of geologic time; a division of a period. (Example: Pleistocene Epoch).

ERAA unit of geologic time that is next in magnitude beneath an eon; consists of two or more periods. (Example: Paleozoic Era).

ESCARPMENTA long, more or less continuous cliff or steep slope facing in one general direction; it generally marks the outcrop of a resistant layer of rocks, or the exposed plane of a fault that has moved recently.

FAULTA fracture surface or zone of fractures in Earth materials along which there has been vertical and/or horizontal displacement or movement of the strata on opposite sides relative to one another.

FELSICA mnemonic adjective derived from feldspar + lenad (feldspathoid) + silica + c, and applied to an igneous rock having abundant light-colored minerals in its mode; also, applied to those minerals (quartz, feldspars, feldspathoids, muscovite) as a group. It is the complement of mafic.

FERROMContaining iron and magnesium; applied to mafic minerals.

FILL[sed] Any sediment deposited by an agent so as to fill or partly fill a valley, a sink or other depression.

FISSILITYA general term for the property possessed by some rocks of splitting easily into thin layers along closely spaced, roughly planar, and approximately parallel surfaces, such as bedding planes in shale or cleavage planes in schist; its presence distinguishes shale from mudstone. The term includes such phenomena as bedding fissility and fracture cleav...

FISSUREA relatively wide planar opening in bedrock that originated as a fracture or fault. The opening may be partially or totally filled with soil or, if open, can act as a conduit for flowing water.

FLAGGYSaid of rock that tends to split into layers of suitable thickness for use as flagstone.

FLOODPLAINThe surface or strip of relatively smooth land adjacent to a stream channel produced by the stream's erosion and deposition actions; the area covered with water when the stream overflows its banks at times of high water; it is built of alluvium carried by the stream during floods and deposited in the sluggish water beyond the influence of the swift...

FLUORAPATITE(a) A very common mineral of the apatite group: Ca5(PO4) 3F. It is a common accessory mineral in igneous rocks. Syn: apatite. (b) An apatite mineral in which fluorine predominates over chlorine and hydroxyl.

FLUVIALOf or pertaining to a river or rivers.

FLUVIOLACUSTRINESedimentary deposits formed by a combination of fluvial (river) and lacustrine (lake) conditions.

FORMATIONThe basic rock unit distinctive enough to be readily recognizable in the field and widespread and thick enough to be plotted on a map. It describes the strata, such as limestone, sandstone, shale, or combinations of these and other rock types. Formations have formal names, such as shale, or combinations of these and other rock types formations have...

FOSSILAny remains or traces of a once-living plant or animal preserved in rocks (arbitrarily excludes Recent remains); any evidence of ancient life. Also used to refer to any object that existed in the geologic past and for which evidence remains (for example, a fossil waterfall).

FRIABLESaid of a rock or mineral that crumbles naturally or is easily broken, pulverized, or reduced to powder, such as a soft and poorly cemented sandstone.

GALENAOrdovician age groups of rock which are largely dolomite with a shaly zone near the middle and some limestone beds in the lower portion.

GEOLOGYThe study of the planet Earth that is concerned with its origin, composition, and form; its evolution and history; and the processes that acted (and act) upon it to control its historic and present forms.

GEOPHYSICSStudy of the Earth with quantitative physical methods. Application of the principles of physics to the study of the earth, especially its interior.

GISComputer software used to input, store, retrieve, manipulate, analyze and output geographically referenced data or geospatial data, often in the form of maps, in order to support decision making for planning and management of land use, natural resources, environment, urban facilities, transportation, and other administrative records.

GLACIATIONA collective term for the geologic processes of glacial activity, including erosion and deposition, and the resulting effects of such action on the Earth's surface.

GLACIERA large, slow-moving mass of ice formed on land by the accumulation, compaction, and recrystallization of snow.

GRADIENTA part of a surface feature of the Earth that slopes upward or downward; the angle of slope, as of a stream channel or of a land surface, generally expressed by a ratio of height versus distance, a percentage or an angular measure from the horizontal.

GRANITEA plutonic rock in which quartz constitutes 10 to 50 percent of the felsic components and in which the alkali feldspar/total feldspar ratio is generally restricted to the range of 65 to 90 percent.

GROUNDWATERWater present below the water table in small, often microscopic, interconnected pore spaces between grains of soil, sand and/or gravel, and in open fractures and/or solution channels in rock.

IGNEOUSSaid of a rock or mineral that solidified from molten or partly molten material, (that is from magma).

ILLINOIANPertaining to the classical third glacial stage of the Pleistocene Epoch in North America, between the Yarmouthian and Sangamonian interglacial stages.

INDURATEDSaid of a compact rock or soil hardened by the action of pressure, cementation, and especially, heat.

IRONTONCambrian age sandstone formations occurring under northern and central Illinois which are clean, white, coarse to fine grained. Neither outcrop in Illinois. Thin dolomite beds may occur in upper part of the Ironton.

JOINTA fracture or crack in rocks along which there has been no movement of the opposing sides (see also Fault).

KANSANPertaining to the classical second glacial stage of the Pleistocene Epoch in North America, after the Aftonian interglacial stage and before the Yarmouthian.

KARSTCollective term for the land forms and subterranean features found in areas with relatively thin soils underlain by limestone or other soluble rocks; characterized by many sinkholes separated by steep ridges or irregular hills. Tunnels and caves formed by dissolution of the bedrock by groundwater honeycomb the subsurface. Named for the region aroun...

KARSTTERAn area or region of the surface of the earth whose landscape is characterized by sinkholes, caves, springs, disrupted land drainage, and an underground drainage system. Karst terrains form in areas with carbonate rock (limestone and dolomite), and areas underlain by other types of soluble rock (for example, salt or gypsum).

KEOKUKThe Keokuk Limestone is named for Keokuk, Lee County, Iowa. At the type locality about 70 feet of Keokuk Limestone overlying the Burlington Limestone is well exposed along Soap Creek and in the Mississippi River bluff near the mouth of the creek. Like the Burlington, the Keokuk is primarily a biocalcarenite. In the type region the lower 30 feet is ...

LACUSTRINEProduced by or belonging to a lake.

LASALLEANTICLINEThe LaSalle Anticlinal Belt is the most prominent anticlinal feature in the Illinois Basin. It is actually a complex structure of en echelon folds, asymmetrical anticlines, and monoclines stretching from Stephenson County in northern Illinois through Lawrence County in southeastern Illinois.

LAURASISA protocontinent of the Northern Hemisphere, corresponding to Gondwana in the Southern Hemisphere, from which the present continents of the Northern Hemisphere have been derived by separation and continental displacement. The supercontinent from which both were derived is Pangea. Laurasia included most of North America, Greenland, and most of Euras...

LAVAMolten, fluid rock that is extruded onto the surface of the Earth through a volcano or fissure. Also the solid rock formed when the lava has cooled.

LIMESTONEA sedimentary rock consisting primarily of calcium carbonate (the mineral, calcite). Limestone is generally formed by accumulation, mostly in place or with only short transport, of the shells of marine animals, but it may also form by direct chemical precipitation from solution in hot springs or caves and, in some instances, in the ocean.

LITHIFYTo change to stone, or to petrify; especially to consolidate from a loose sediment to a solid rock.

LITHOLOGYThe description of rocks on the basis of color, structure, mineral composition, and grain size; the physical character of a rock.

LOCALRELIEFThe vertical difference in elevation between the highest and lowest points of a land surface within a specified horizontal distance or in a limited area.

MAFICSaid of an igneous rock composed chiefly of one or more ferromagnesian, dark-colored minerals in its mode; also, said of those minerals. The term was proposed by Cross, et al. to replace the term femag, which they did not consider to be euphonious. Etymol: a mnemonic term derived from magnesium + ferric + ic. It is the complement of felsic.

MAGMANaturally occurring molten rock material generated within Earth and capable of intrusion into surrounding rocks or extrusion onto the Earth's surface. When extruded on the surface it is called lava. The material from which igneous rocks form through cooling, crystallization, and related processes.

MAQUOKETAThe Maquoketa Shale Group is Ordovician in age, with green to blue shale with limestone and dolomite beds in lower portion. The group has been eroded over part of northern Illinois, but underlies most of the rest of the state.

MEANDEROne of a series of somewhat regular, sharp, sinuous curves, bends, loops, or turns produced by a stream, particularly in its lower course where it swings from side to side across its valley bottom.

MEANDERSCARSCrescent-shaped, swales and gentle ridges along a river's flood plain that mark the positions of abandoned part of a meandering river's channel. They are generally filled in with sediments and vegetation and are most easily seen in aerial photographs.

MESOZOICAn era of geologic time, from the end of the Paleozoic to the beginning of the Cenozoic, or from about 248 to about 65 million years ago.

MINERALA naturally formed chemical element or compound having a definite chemical composition, an ordered internal arrangement of its atoms, and characteristic crystal form and physical properties.

MISSISSIPPIANA period of the Paleozoic era (after the Devonian and before the Pennsylvanian), thought to have covered the span of time between 354 and 323 million years ago; also, the corresponding system of rocks. It is named after the Mississippi River valley, where rocks of this age are well exposed. It is the approximate equivalent of the Lower Carboniferou...

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